Electrolytic process for selective acyloxylation



United States Patent 3,448,021 ELECTROLYTIC PROCESS FOR SELECTIVE{ACYLOXYLATION William J. Koehl,-Jr., Yardley, Pa., assignor to MobilOil Corporation, a corporation of New York No Drawing. Filed Dec. 28,1966, Ser. No. 605,225 Int. Cl. B01k 1/00; C07c 67/00 US. Cl. 204-72 10Claims ABSTRACT OF THE DISCLOSURE A method for the electrochemicalpreparation of acyloxy derivatives of substituted benzene compounds. Forexample, in the electrochemical acetoxylation of toluene, xylenes, etc.,the use of a cobalt acetate catalyst produces benzyl acetates to theexclusion of phenyl acetates.

BACKGROUND The electrochemical acyloxylation of toluene, xylene andother substituted benzene compounds has heretofore produced mixtures ofphenyl and benzyl esters in varying porportions. For example, incopending patent application 399,370 (now US. Patent No. 3,347,758),filed Sept 25, 1964 now Patent No. 3,294,974, by the present applicant,the acetoxylation of para-xylene is disclosed. The two main productswere 2,5-dimethylphenyl acetate and 4-methylbenzyl acetate. In the fivenuns reported in said copending application, directed to theacetoxylation of para-xylene under different conditions, the productionof 2,5-dimethylphenyl acetate varied from 51 to 40% and the productionof 4-methylbenzyl acetate varied from 49 to 43%. Thus it had previouslybeen concluded that the electrochemical acyloxylation of substitutedbenzene compounds was inherently nonselective. A concurrently filedpatent application (Serial number 606,520) by the present applicant isdirected to the electrochemical acyloxylation of olefins.Non-electrochemical acyloxylation is disclosed in U.S. Patent 3,228,971.

THE INVENTION It is an object of this invention to provide a method forthe acyloxylation of substituted benzenes to produce the correspondingbenzylic esters with a high degree of selectivity.

It has now been found that this objective can be achieved by conductingthe acyloxylation reaction in the presence of promoters, such as cobaltacetate, preferably in the presence of a minor amount of water. Thisinvention therefore comprises electrolyzing in the presence of inertelectrodes a solution comprising an alkyl substituted benzene compound,an electrolyte, an acyloxy group, and a selectivity promoter. Thepresence of a solvent is preferable.

In more detail, the substituted benzene compound may 7 be depicted asRID wherein R is a low molecular weight alkyl snbstituent having 1 to 12carbon atoms, n is an integer from 1 to and R is a substituent selectedfrom the class consisting of hydrogen H--, alkyl R--, mononuclear arylAr--, alkoxy RO, phenoxy PhO--, acyloxy RCOO, halogen, such as chlorine,cyano NC-, carboxy R"OOC and acylamido R"CONH groups, wherein R is asdefined above. The preferred substitueuts for R and R' are alkyl groups,having up to about 10 or 12 carbons, and preferably up to 6 or 4carbons, and comprising straight or branched chain radicals. Themononuclear aryl groups are radicals like phenyl, tolyl, and the like.The R" radical may be alkyl, substituted alkyl, phenyl, or substitutedphenyl, and the number of carbons in the alkyl or substituted alkylgroups may be described above. In all of the foregoing substituents, thesubstituent itself may have a substituting group of the kind described,i.e., an alkyl, mononucle-ar aryl, alkoxy, phenoxy, acyloxy, oracylamid-o group.

Specific compounds that may be converted to acyloxy derivatives includesuch alkyl benzenes as toluene, the xylenes, hemimellitene,pseudocumene, mesitylene, prehnitene, durene, isodurene,pentamethylbenzene, hexamethylbenzene, ethylbenzene, the variousdiethylbenzenes and polyethylbenzenes, propylbenzene, cumene, cymene,butylbenzene, etc., also alkyl benzenes with substituted alkyl sidechains like (Z-methoxymethyDbenzene; and tolylbenzene; alsoalkoxyalkylbenzenes such as p-tolyl methyl ether; alsoacyloxyalkylbenzenes such as acetoxytoluenes; andacylamindoalkylbenz-enes like acetotoluides.

The functions of solvent, electrolyte and acyloxy group may be obtainedfrom one or more compounds. For example, a combination of an alkanoicacid and its alkali metal salt satisfies all three of the abovefunctions in this invention. The alkanoic acid is a suitable solvent,its alkali metal salt is an electrolyte, and each may provide acyloxygroups for reaction with the benzylic compounds.

Suitable alkanoic acids which function as a solvent for the othercomponents of the solution and also for various products of electrolysisare the C-1 to C-lO acids, and preferably the 0-2 to C-6 acids such asacetic, propionic, butanoic and pentanoic and their isomers, and thevarious hexanoic acids. Branched as Well as straight chain acids areuseful, including such acids as Z-methylbutyric, 3-methylbutyric, andtri-methylacetic. Other useful solvents are acetonitrile anddimethylforamamide. In general any solvent or solvent mixture may beused which dissolves the reactants, and which of course does not itselfinterfere with the desired reaction. Although the alkanoic acids have alow degree of ionization they may also serve as a source of electrolyteby the addition of a base to form the corresponding salts.Alternatively, the alkali metal salt of the above used acid or another,which may serve as a re actant and electrolyte, can be separately added.The salt is usually the sodium or potassium salt, although lithium andquaternary ammonium salts are suitable.

The use of a minor proportion of water, 0.5 to 2% of the total charge,is preferred. However, as illustrated in the examples this is notcritical, since anhydrous solutions or substantial amounts of water maybe used. Minor amounts of water improve the selectivity.

, Any suitable inert cobalt salt may be used in this invention, and itmay be added in the cobaltous or cobaltic state, the former beingpreferred because of its availability. The cobalt compound may be formedfrom any of the above alkanoic acids, the preferred promoter is cobaltacetate Co(C H O -4H O. Examples of inorganic cobalt salts are Co(ClOand C080,. Other metals which may be chosen are those which undergo aone electron redox reaction at a potential of 1.0 to 2.0 volts such ascerium or manganese. These metals may also be used in the form of anysoluble salt, inorganic or organic, such as the acetate, or from otheralkanoic acids. The electrodes may be carbon or graphite, or formed fromany inert metal such as copper, stainless steel, platinum, silver,nickel, lead, etc. The anode is preferably carbon, platinum or gold,whereas the cathode may be any of the above. Forms of the electrodes areconventional.

The composition range of the solution to be electrolyzed may varyWidely, suitable ratios are solvent, lzsubstituted benzene, 0.05-1:alkanoic acid salt, 0.05-1zwater, .005.5: and promoter, .005-5.

The current density may be maintained over a fairly wide range, say0.001 to 1.0, and more preferably 0.01 to 4 and the selectivities forbenzyl acetate formation are smaller.

0.26, a mpJ sq. cm. As is known, the current density value TABLE Hdetermines the rate or speed of the electrolysis. Applied t voltage issupplied by any suitable DCsource. 5

Room temperatures are preferred, e.g., 20 to 40 C., Reaetant Product3Percellt al g higher temperatures are useful. p to the boiling 11m-Chlorotoluene,14 m m-Chlorobenzyl acetate "52 point of the solution,If desired, a diaphragm of conven- 12 po achlorobenzyl acetate tionalmaterial may be used to separate the cathode from 13 Phenylacemmtme' 20ggfigf P the anode .in order to prevent possible reaction of the prod-1O 14 m-CYan0t01uene.10 gm m-cyanobemyl acetate ucts formed at oneelectrode with those at the other. Agita- 15 p'cymotoluene'p'cyanobenzyl acetate tion is desirable but can be omitted. The pH ofthe electrolyte solution may initially be on the acid side, or neutral,g g i z i i q g g g ifi gi but preferably is on the acid side, andsuitably may range Pg fi 0 Pf. 1 1S g e um a i from a pH of 4 to 6, morebroadly from 2 to 7. pai t ir r g frz r n $5253: 2 1 1 21 sczp g f thisin v ent io n Z; p Invention may be illustrated by the followmgexamthose skilled in the art will readily understand. Such variationsand modifications are considered to be within the EXAMPLES 1-7 purviewand scope of the appended claims.

In the light of the foregoing description, the following Severalexperiments with toluene are summarized in is claimed:

Table I. The reactants in each run included 20 ml. toluene 1. A methodwhich comprises electrolyzing a solution and 20 ml. of potassiumacetate. In all reactions the anode comprising an electrolyte, asolvent, a promoter selected was carbon, the cathode was platinum. Acur-rent of 0.25 from metal compounds which undergo a one electronreamp., 0.025 amp/cmf", at 5 to 7 v. was used for about 6 dox reactionat a potential of 1.0 to'2.0 volts, and a subhours, and the temperaturewas to C. The reacted 25 stituted benzene compound of the followingformula: mixture was added to a solution of 125 g. of sodium chlorideand 6 g. of ferrous sulfate in 400 ml. of water and the organic productsextracted with ether. Analysis was by vapor phase chromatography.

The products include benzyl acetate, benzaldehyde, and 30 btbenzyl. Whenthe solution 1s anhydrous or when the cowherein R is a low molecularWeight alkyl substituent haw balt acetate concentration 1s too small,tolyl acetates as m 142 carbon atoms, is an integer f 1 to 5, and wellas xylenes are also formed. When the data obtained is a substituentSelected f the class Conisting f using cobalt acetate are compared withdata obtained withdrdgen alkyl mononucleara Q alkoxy ggltrcolastlt,d run7, the catalytic effect of the cobalt can be 0 phgnoxy PhO acyloxy oohalogen cyano Although the cobalt was a d as fk f groups, wherein R isas defined ab ve, and R" is a sub- Cobalt w formed duflng h l'eactlon-Tltratlon 0f stituent selected from the group consisting ofalkyl, subsamples taken from the cell showed that the cobalt (III) 40 stit t dalkl, phen l, or substituted phenyl. concentration had increased graduallyto about 25% of the 2 The process of claim 1 wherein said promoter is atotal cobalt when the reaction was stopped. cobalt compound of analkanoic acid.

TABLE I V Reactants 7 Current efficiency, percent Benayl acetate In1110110- HOAc, C0(OA0)1.4H30, Mono- Benzacetate, Example ml. AczO,m1.1120, m1. g. acetate aldehyde Xylene Bibenzyl percent 10 61 5.6 1.8 1005 54 4.2 1.8 100 1 26 1.4 3.7 0.9 1 92 5 30 2.6 1.6 100 10 27 2.1 5. s0. 9 t 93 5 24 2.0 5.3 0.7 94 None 2 2. 5 14 0. 4 26 l The remainder isoand p-to1y1 acetates. 3 Includes tolyl acetates.

EXAMPLES 8-9 3. The process of claim 1 wherein said solvent is an Usingthe same concentrations as in Example 1, m-xylalkanolc and havmg carboni ene yielded 3-methylbenzyl acetate with a current elfii Process of dam3 whemm sald alkanolc and ciency of 46%. p-Xylene (same conditions asExample 2) 1S acetlc acld gave 4-methylbenzyl acetate, 62% currentefiiciency. No 1. Process of Glam 1 F said .electmlyte 15 othermonoacetates were detected in either case. Small an a ka 1 metal salt ofan alkanolc and havmg 1 10 amounts of other compounds including methylbenzaldebon atoms hydes and diacetates were found 6. The process ofclaim 5 whereln satdelectrolyte is 65 an alkalr metal salt of aceticacid. EXAMP 10 7. The process of claim 1 wherein said solution containsEthylbenzene was reacted under the same conditions as to 2% Water' aExample 2. The products were 1-phenylethyl acetate and The P of Glam 1 wthe solutlo-n Includes acetophenone with urr emciencies of 59% and 6%.the following ratios of compounds: solvent, lzsubstttuted 7O benzene,0.05-1:electrolyte 0.05-lzwater 0.005-..5:and EXAMPLES 11-15 promoter0.005.5.

Table II presents the data for a series of examples in Thfi Process ofclaim 1 Whfifein the electrolysis is which the procedure employed wasthg ame as that for carried out With it CUIIfiIlt density Of 0.001 101.0 amp./ Example 2. In each of these examples cobalt is essential. sq.cm.

Without cobalt the current eflicieucies for acetoxylation 10. Theprocess of claim 1 wherein R and R are low .5 6 molecular weight alkylsubstituents having 1-6 carbon 3,252,878 5/1966 Koehl 20459 atoms, andn. is the integer from 1 to 3. 3,397,2 6 8/1968 Fenton 20478 JOHN H.MACK, Primary Examiner. H. M. FLOURNOY, Assistant Examiner.

References Cited UNITED STATES PATENTS U.S. Cl. X.R. 3,252,876 5/1966Koehl 204-59 5

